Summary of work The protein p21 (Cip1, Waf1, Sdi1) is a potent inhibitor of cyclin dependent kinases (CDKs) and cyclins. P21 can also block DNA replication through its interaction with the proliferating cell nuclear antigen (PCNA) which is an auxiliary factor for polymerase delta;. In addition to its role on replication, PCNA is clearly implicated in the repair resynthesis step of nucleotide excision repair (NER). Since PCNA particpates in both DNA repair and replication, it has been speculated that p21 might also regulate NER through its interaction with PCNA. Previous studies on the role of p21 on NER have yielded contradictory results that make it difficult to reach a general consensus with regard to the precise role of p21 in NER. In this study, we have investigated the effect of p21 on NER both in vitro and in vivo using purified fragments of p21 containing either the CDK- binding domain or the PCNA-binding domain of the protein. In the in vitro studies DNA repair synthesis was measured in extracts from normal human fibroblasts using plasmids damaged by ultraviolet (UV) irradiation. A permeabilized cell system and electroporation of intact cells were utilized for in vivo studies. The results show that the C-terminal domain of the p21 protein, which binds to PCNA, inhibits NER both in vitro and in vivo. A 50% percent inhibition of in vitro NER occurred at a ratio of 50:1 p21 C-terminus to PCNA monomer. Our results suggest that the inhibition occurs at the resynthesis step of the repair process. We further demonstrate that the inhibition of DNA repair is mediated via binding of p21 to PCNA since it is relieved by addition of purified PCNA protein.